(1) Field of the Invention
The present invention relates to the improvement of a replenishment method of a magnesium-based compound which can be used in a magnesium hydroxide desulfurization method for fixing and removing sulfur oxides contained in an exhaust gas by the use of the magnesium-based compound as a desulfurizing agent; and the improvement of a double decomposition method for fixing the sulfur oxides by the use of the magnesium-based compound and then double-decomposing, with a basic calcium compound, a solution of a desulfurizing step which has passed through an oxidizing step to form magnesium hydroxide and gypsum dihydrate (hereinafter refrred to as xe2x80x9cgypsumxe2x80x9d).
(2) Description of the Prior Art
A magnesium hydroxide desulfurization method is well known which comprises a desulfurizing step of removing sulfur oxides from an exhaust gas by bringing a magnesium-based compound (such as magnesium hydroxide, magnesium oxide, magnesium carbonate, or magnesium hydroxide obtained using dolomite and the like as raw materials) into gas-liquid contact with the exhaust gas, an oxidizing step of converting magnesium sulfite and the like contained in the solution of the desulfurizing step to magnesium sulfate by oxidizing the solution, and a double decomposition step of subjecting magnesium sulfate to a double decomposition reaction with a basic calcium compound to produce magnesium hydroxide and gypsum. Furthermore, in the known method, (1) magnesium hydroxide and gypsum obtained in the double decomposition step are sent to the desulfurizing step directly or via a storage step in which magnesium hydroxide and gypsum are temporarily stored, and contained magnesium hydroxide is used as a desulfurizing agent again and gypsum is recovered from the desulfurizing step or the oxidizing step; or (2) a slurry of magnesium hydroxide and gypsum obtained in the double decomposition step is separated into a slurry having a higher magnesium hydroxide concentration and a slurry having a higher gypsum concentration, and the former is sent to the desulfurizing step in which contained magnesium hydroxide is used as the desulfurizing agent again, while the latter is sent to a gypsum separation step in which gypsum is separated and collected.
In this magnesium hydroxide desulfurization method, magnesium hydroxide which is used as the desulfurizing agent is circulated in the system while its form changes into magnesium sulfite, magnesium sulfate, and magnesium hydroxide again. However, in order to prevent the accumulation of dust, chlorine compounds and the like in the exhaust gas, a part of the solution is required to be discharged as a blow water, so that a part of the magnesium compound which circulates in the system is discharged with this blow water, which means the loss of the magnesium compound.
Heretofore, in order to replenish the system with magnesium hydroxide for lost magnesium, 30 to 35% by weight of a magnesium hydroxide slurry is fed to a recovery magnesium hydroxide tank via a line L1, as shown in FIG. 3.
On the other hand, the magnesium compound contained in the blow water discharged from the system varies in concentration at positions where it is drawn. However, even if the blow water is drawn at any position, a magnesium compound concentration in the blow water is much lower as compared with a concentration of magnesium hydroxide to be replenished. Therefore, during the operation for a long time, a balance between magnesium and water in the system changes, which fact makes the continuous operation impossible. To avoid such an inconvenience, a recovery magnesium hydroxide tank is provided as a kind of buffer tank, but even in this case, a water level in the recovered magnesium hydroxide tank changes very noticeably, and so it is difficult to accomplish the stable operation.
The double decomposition step which is one step of the magnesium hydroxide desulfurization method has been conventionally carried out by a device shown in FIG. 7. This device comprises a double decomposition tank 4 and a basic calcium slurry tank 5.
The double decomposition tank 4 comprises an inner cylinder 6 which partitions the tank into an inner part and an outer part and which does not reach the bottom of the double decomposition tank, a means for circularly feeding, to an upper position of the double decomposition tank 4, a part of a double-decomposed slurry (hereinafter referred to as xe2x80x9ccirculating fluidxe2x80x9d) drawn through the bottom of the tank 4, and a discharge nozzle 7 for overflow disposed at an upper side of the outer part of tank 4.
The slurry of the inner part of the inner cylinder 6 of the double decomposition tank is stirred by a stirrer to promote the growth of gypsum crystals. On the other hand, owing to the structure of the inner cylinder, the slurry of the outer portion of the inner cylinder is not influenced by the stirring.
A slurry containing magnesium hydroxide as a main component is drawn through the discharge nozzle 7 for overflow disposed at the upper part of the double decomposition tank, and a slurry containing gypsum as a main component is drawn through the bottom of the tank.
In the basic calcium slurry tank (hereinafter referred to as xe2x80x9cthe slurry tankxe2x80x9d sometimes), a basic calcium compound and water or a separated fluid of a gypsum dehydrator or the like are used to prepare a slurry, and this slurry is then fed to the double decomposition tank. At this time, the slurry is fed to the inner part in the double decomposition tank.
Thus, in preparing the basic calcium slurry by the conventional double decomposition method, the basic calcium compound and water are added to the slurry tank to prepare a slurry having a predetermined concentration, and the thus prepared slurry is fed to the double decomposition tank. Consequently, water used for the preparation is required to be discharged as the blow water. In this case, depending on a position where the blow water is discharged, the magnesium compound is discharged in the form of magnesium sulfate or magnesium hydroxide together with the blow water, which means the loss of the magnesium compound.
Further, in the conventional method in which the circulating fluid is fed to the inner part of the double decomposition tank, the magnesium hydroxide slurry produced by the double decomposition reaction is discharged through the discharge nozzle for overflow with an upward flow via the outer part of the double decomposition tank. The magnesium hydroxide slurry is not steadily but intermittently discharged like breathing by this overflow. Therefore, there is observed a phenomenon that properties of the slurry drawn through the double decomposition tank, for example, a content of magnesium hydroxide and a ratio between magnesium hydroxide and gypsum change.
An object of the present invention is to provide a replenishment method of a magnesium-based compound for constantly maintaining a magnesium/water balance which is the problem of the above-mentioned conventional technique. Another object of the present invention is to provide a method for solving the problem of the above-mentioned double decomposition step.
The first aspect of the present invention is directed to a replenishment method of an absorbent in a desulfurization method of an exhaust gas which comprises the step of feeding magnesium hydroxide to make compensation for a magnesium-based compound which is incorporated in a blow water and then discharged from the system, while a ratio of supplementary magnesium hydroxide to be fed is adjusted to a molar ratio of total magnesium/water in the system.
The second aspect of the present invention is directed to a double decomposition method which comprises the step of using a double decomposition tank having an inner cylinder which partitions the double decomposition tank into an inner part and an outer part and which does not reach the bottom of the double decomposition tank, a discharge nozzle provided at a peripheral upper position of the double decomposition tank, and a means for circulating a part of a slurry drawn through the bottom of the tank to an upper position of the tank, whereby the part of the slurry drawn through the bottom of the double decomposition tank is fed to an upper outer part in the double decomposition tank.
The third aspect of the present invention is directed to a double decomposition method which comprises the step of using the double decomposition tank, whereby a part of the slurry of the double decomposition tank is drawn through the discharge nozzle or another discharge nozzle disposed at a position lower than the discharge nozzle, and then fed to a basic calcium slurry tank for preparing a basic calcium compound to be fed to the double decomposition tank, and the basic calcium compound is mixed with the part of the slurry of the double decomposition tank to prepare a slurry of the basic calcium compound.
According to the replenishment method of magnesium hydroxide, a water balance in the system is maintained within a predetermined range. Furthermore, according to the double decomposition method, the properties of the magnesium hydroxide slurry discharged from the double decomposition tank can be made homogeneous, and the water balance in the system can be maintained, which contributes to the stable operation of a desulfurization method with magnesium hydroxide.